As well known, an electrolytic capacitor is used in a main circuit smoothing section of a voltage source inverter device. Such an electrolytic capacitor has a limited life. Usually, when the designed time of an inverter device has elapsed, it is therefore deemed that the life of an electrolytic capacitor has expired, and the electrolytic capacitor is replaced with a new one. However, the life of an electrolytic capacitor is largely varied depending on the environment in which an inverter device is used. Therefore, there are a case where, although an electrolytic capacitor has not yet deteriorated, the capacitor is replaced with a new one after the designed constant time has elapsed, and an opposite case where, although an electrolytic capacitor has deteriorated, the life of the capacitor is not deemed to expire and hence the capacitor is not replaced because the designed constant time has not yet elapsed.
An inverter device in which the life of an electrolytic capacitor used in the inverter device can be determined is disclosed in Patent literature 1 (JP-A-11-98854). The inverter device which is disclosed in Patent literature 1, and in which an electrolytic capacitor is used in a main circuit smoothing section comprises electrolytic capacitor life determining means having: a discharge resistor which is connected in parallel to the electrolytic capacitor; a voltage detecting section which monitors the voltage between the terminals of the electrolytic capacitor; and a judging section which, when a voltage supply to the electrolytic capacitor is interrupted, measures the discharge time constant t (t=C·R) that is determined by the discharge resistance R and the electrostatic capacitance C of the electrolytic capacitor, and which, when an electrostatic capacitance calculated from the measured value exceeds an allowable range of the electrostatic capacitance of the electrolytic capacitor that is previously obtained as a reference, judges that the life of the electrolytic capacitor has expired.
A capacitor life diagnostic apparatus which is developed for the purpose of accurately diagnosing the life of a capacitor is disclosed in Patent literature 2 (JP-A-11-231008). In Patent literature 2, at a timing after at least one of application of a power source voltage and stop of the application, the voltage between terminals of a capacitor is sampled at plural points, a time constant T is obtained from the sampled voltages, the capacitance C0 of the capacitor is calculated from an expression of T/R with using a known resistance such as a resistance R of a rush current preventing resistor that is connected to the capacitor of a switching power source apparatus, and, when it is judged that the capacitance C0 of the capacitor is equal to or smaller than the theoretical worst capacitance Cr=T/(R±ΔR)+ΔC in which a change ±ΔR of the resistance R of the resistor and a change +ΔC of the capacitance of the capacitor caused by a change in ambient temperature are considered, it is judged that capacitance reduction occurs in the capacitor.
A main circuit power source discharging method in which a charging residual voltage of a capacitor is discharged as a load loss by supplying a current to an electric motor through a closed circuit including the motor-without using a dedicated discharging circuit having a discharge resistor is disclosed in Patent literature 3 (JP-A-11-89264). In Patent literature 3, when a DC input power source to a main circuit is interrupted, the control is transferred from a motor driving control to a discharge mode, and a current is supplied to the motor by forming a closed circuit including the motor, whereby the charging voltage of the capacitor is consumed and discharged as a load loss or a torque energy of driving the motor and the load.
In Patent literature 1 or Patent literature 2, since a resistor (the discharge resistor in Patent literature 1, and the rush current preventing resistor in Patent literature 2) is used in the estimation of the life of an electrolytic capacitor, there is a problem in that, when the resistance is varied by a change of the temperature surrounding the resistor, the estimation of the life of an electrolytic capacitor based on the measurement of the time constant hardly maintains the accuracy.
There are other problems as follows. When a resistor to be used for discharging of an electrolytic capacitor is set to have a small heat capacity, the resistance is increased, and hence the output current of the capacitor is reduced. Therefore, the measurement of the discharging time is prolonged. By contrast, when the resistance is reduced in order to shorten the measurement time of the discharging time, a larger power is consumed in the resistor, and hence a resistor of a larger heat capacity must be disposed. Therefore, the inverter device is increased in size.
The resistor to be used for discharging of an electrolytic capacitor is placed adjacent to the electrolytic capacitor. Even when heat of a quantity within a usable range of the resistor is generated, therefore, the temperature of the electrolytic capacitor itself is raised by the heat, thereby producing a further problem in that the heat causes the electrostatic capacitance of the electrolytic capacitor to be varied.
The invention has been conducted in order to solve these problems. It is an object of the invention to obtain an inverter device in which the life of an electrolytic capacitor can be accurately determined, and the timing of replacement of the electrolytic capacitor can be exactly determined.